The invention relates to lighting control systems, and more particularly to electronic control apparatus for controlling the intensity of a plurality of electric lamps of the type used for illumination in theatres, television studios and the like.
Lighting systems are known wherein the intensity of a lamp is controlled by a thyristor in series with the AC line and the lamp load. The firing angle of the thyristor controls the power to the lamp and hence the intensity of the lamp in response to a small analog DC control voltage signal. Systems are also known wherein the said DC control signal is derived from a binary encoded signal by a digital to analog converter means. Such systems require the use of a digital to analog converter and an analog control circuit for each channel to be controlled in order to provide a phase encoded trigger pulse in response to the binary control signal. The analog components required in circuits of this type are expensive, subject to variation with respect to time and variation from component to component and are also subject to relatively frequent failure. In addition circuits of this type require frequent adjustment in order to maintain accurate and uniform transfer characteristics from dimmer to dimmer. A further disadvantage of prior art systems is that because of the expense of each dimmer and the number of lighting circuits to be controlled a relatively small number of dimmers are provided and a "patch panel" is additionally provided to connect these dimmers to the several circuits.
It is a general object of the present invention to provide a lighting control system which is more accurate and reliable than systems heretofore available.
Another object of the present invention is to provide a lighting control system wherein the marginal cost of dimmers in the system is less than that of such systems heretofore available.
Another object of the present invention is to provide a lighting control system wherein the transfer characteristic may be easily and accurately changed yet will not require frequent adjustment.
Briefly, these and other objects of the invention are achieved by a system comprised entirely of digital means. A binary encoded signal indicating the desired intensity for each control channel as a number of intensity increments is provided by a digital computer or similar digital signal generator. A trigger pulse generator means produces one trigger pulse for each channel, the phase angle of said pulse being a function of the binary encoded intensity indicating signal. The transfer characteristic is controlled by a time-base generator which sequentially counts out binary values stored in a memory to generate a phase encoded pulse train which effectively divides each half-cycle into a number of timed increments corresponding to the intensity increments defined by the intensity indicating signal. The intensity indicating signal and the time-base signal are combined in the trigger pulse generator to produce a trigger pulse which, when applied to the gate of a thyristor, controls the intensity of a lamp connected in series with said thyristor. The invention also includes a novel method for compensating for changes in line voltage by providing a voltage controlled clock frequency to the time-base generator.